use empty string instead of none for no devices
no primary device should show as empty string and not null
show user voicemail in edit form
Delete voicemail account if field submitted empty

We delete old voicemail object for the following
conditions:

1. Old object exists  AND
2. We change the voicemail number.

In the future we need an autocomplete feature in the field
to check in realtime if that voicemail number is already
being used. Form validation should not allow submission
if voicemail is already being used.
Add voicemail account if field submitted with id

We add new voicemail account for the following
conditions:

1. new number is not equal to old number. AND
2. New number is not empty string

We know that we already deleted old object in previous
operation for this case so it's safe to create the voicemail
account here once we know the voicemail number is available.
We will check availability in future using autocompletion
in the form together with form validation.